Disposal of organic waste at sea, particularly bilge water containing oil from the engine room, is a serious problem governed by state, national and international environmental regulations. Even were a sea-going vessel able to dump its holding tanks at sea, it would have to leave port to do so. Since the day-to-day operation of a large ship such as an ocean liner, cruise ship, or battleship is closely tied to the successful management of its solid and liquid waste, the importance of the problem will be better appreciated when one considers that a large ship's waste generation at sea is much like that of a small city, except that the ship rolls, pitches and tosses, and there is not much space available to store the waste for disposal at a later date.
This invention is directed to the solution of a specific problem, namely the disposal, on-board ship, of liquid waste with a high solids content. The solids are both suspended (or, water-insoluble), and dissolved. The constant motion of a marine vessel, at times less violent than at others, precludes the successful use of an organic waste liquid disposal system in which separation of suspended solids and activated sludge is effected in a biological reactor ("bioreactor") in combination with a conventional gravity separation means for separating sludge. Even when an ultrafiltration (UF) system has been used to separate oil/water-emulsions with appropriate membranes, in combination with conventional oil removal techniques, the UF system was found ineffective to remove lower molecular weight hydrocarbons, say in the range from C.sub.6 -C.sub.12, and highly soluble organics such as surfactants in the grey water. Organic compounds which cannot be separated by UF membranes must be removed by biological oxidation.
The problem of disposing of raw sewage onboard ship was addressed by the U.S. Navy in a report titled "Phase I Final Report--Shipboard Sewage Treatment System--Contract No. N00024-71-C-5329" published by National Technical Information Service in report No. AD733082. In this system, an aerated membrane biological reactor ("MBR") was used to treat (or "condition") the raw sewage stream. Activated sludge was continuously withdrawn from the reactor through a rotating, self-cleaning drum type screen that prevents cloth, paper, plastic, metal, or wood pieces from entering the membrane system. The conditioned and screened sludge is then circulated to the membrane loop at a rate approximately four times that of the incoming sewage. Some of the conditioned sewage passes through the membrane as purified effluent (permeate) and is discharged, while the remainder (concentrate) is recycled to the reactor. Though they treated the raw sewage successfully, they concluded that activated sludge systems are highly responsive to changes in environmental conditions within the MBR. This sensitivity of the MBR dissuaded one from dealing with a more complicated system, namely one in which other streams might be added to the raw sewage.
Though there is no suggestion in the Navy report to add any other stream to the shipboard raw sewage MBR, in the past, oily streams have been treated in a raw sewage MBR on land. A treated stream is withdrawn from the reactor at the same rate as the incoming feed, and filtered through a cross-flow UF membrane. Digestion of the oils required the addition of nutrients for the organisms, typically, phosphates and nitrogen-containing compounds.
There was no suggestion that any commonly generated waste stream might provide adequate nutrient value. Moreover, with respect to grey water, there was no reason to treat such a waste stream which regulations permitted to be dumped at sea without treatment. Further, depending upon whether solid or liquid nutrients were provided, the operation of the reactor was different. Since water-insoluble oil in a bilge water stream typically discharged from a ship's engine room twice a day, is peculiar to a large ship, and there was no suggestion in the art as to what might provide an adequate source of nutrients, it was particularly unexpected that adequate nutrition for effective microorganisms, including bacteria, may be provided by any combination of streams available on-board a ship.
The discovery that raw sewage ("black water") and waste water from showers, sinks, laundries and kitchen facilities ("grey water"), together, are able to provide the necessary nutrients for appropriately acclimated microorganisms which digest the bilge water, and the availability of both black and grey water on-board ship, sparked the attempt to adapt a MBR for use on-board a ship. It must be kept in mind that combining the bilge water with black and grey water results in a stream having a solids concentration (mass/unit volume of stream), which is from 3 to 10 times greater than that normally encountered in a municipal waste stream.
More specifically, because of the discovery referred to hereinabove, this invention is based on the essentially concurrent treatment and disposal of (i) "black water", (ii) "grey water", and (iii) "oily water" discharged from engine rooms and in ballast water.
A solution to the problems relating to minimizing the amount of activated sludge generated despite variations in the availability of each stream under different conditions of operation of the ship, and how these conditions affect the reliable operation of an on-board MBR, define the invention claimed herein.
Though MBRs are known for use on land-based waste disposal systems, their use on-board ship was deemed an unlikely application because an MBR process to digest insoluble solids, present in a "high-solids" waste stream containing up to 5% organic solids, requires a long solids retention time ("SRT") to provide adequate retention of emulsified oil and certain soluble components, so as to achieve the desired treatment of waste.
A practical shipboard MBR to treat an aqueous suspension of a liquid waste combination of black water, grey water and bilge water, is necessarily compact because it is limited by the availability of space below deck. Yet it must, under full load operating conditions, treat a "high-solids" liquid waste stream having at least 1% organic solids, more typically from 2%-5%; and, this stream of exceptionally high organic content is delivered in surges which, most of the time, load the MBR to its maximum capability.
The solution to the problem requires dealing with two inter-related conversion processes. First, the organic solids are to be converted biomass; and, then, the biomass is to be destroyed by the microorganisms, converting the biomass to carbon dioxide and water.
Since the two processes, simultaneously occurring in the bioreactor are at cross-purposes, operation of the process is carried out under conditions suitable for both processes. Such conditions demand a large excess of oxygen supplied to the live microorganisms under conditions which make the oxygen available to them in such a way as to regenerate themselves and at the same time, destroy themselves. To provide a relatively short hydraulic retention time ("HRT") less than 24 hr, and a very long solids retention time ("SRT") more than 5 days, preferably &gt;10 days, the oxygen must be adequately dispersed in the bioreactor with sufficient macromixing of the activated sludge, so as to do both. The combination of an external gas micronizing means ("micronizer" for brevity) which generates microbubbles, and, an auxiliary stream to supply air, provides macromixing and the desired combination of short HRT and long SRT. Whether the auxiliary stream is air alone, or air entrained in liquid, it provides macrobubbles with sufficient kinetic energy to effect macromixing.
To cope with black water: At the present time, in the art of conforming to the environmental regulations for disposal of liquid waste at sea, black water is stored in holding tanks. A high chlorine level is maintained in the tanks to kill living organisms. To minimize the volume of black water stored, a vacuum system is used to flush toilets. In those instances where a small conventional biological reactor has been retrofitted on-board ship, the reactor was able to treat only the black water because it was too small to handle the volume of grey water. When generation of the raw sewage decreased greatly, as when most of the persons on board debarked, the organisms in the reactor failed to survive. Further, it was found to require too much of an operator's time when it was in operation.
To cope with grey water: to date, it is not treated since there are no regulations which proscribe dumping the untreated grey water at sea.
To cope with oily water: disposable porous substrate filter cartridges, in combination with other oily water separators, have been used because settling tanks are ineffective. Separated oil is then held in a storage tank and the oil-laden cartridges are stored in bins until they can be off-loaded on land.
Currently, there is no system available for use on a sea-going vessel, or even on an oil derrick operating offshore, which system can dispose of oily water, black water and grey water, with due concern for the environmental regulations now in force, or those which are scheduled to be enacted in the near future.
Conventional microfiltration ("MF") and/or ultrafiltration with membranes, not only avoids the time penalty of gravity settling technology, but also provides a highly effective purification means. What was not appreciated is that the permeate is typically less than 5% by volume of the feedstream flowed over the membranes so that the kinetic energy remaining in the concentrate is substantial. It is this remaining kinetic energy which is utilized in a membrane bioreactor system with an in-line gas micronizer such as is disclosed in the parent application.
The rate of transfer of oxygen limits the biomass concentration in an activated sludge wastewater treatment system (see Aerobic biological Treatment of Wastewaters: Principles and Practice by A. W. Bush Pg. 285-312 Oligodynamics Press 1971). There are numerous references teaching how to aerate a bioreactor (hereafter "reactor"); and, membrane devices have long been known to be highly efficient separating means to filter solids-free permeate from the solids-containing concentrate. But aerating a reactor efficiently is not simply a matter of blowing copious amounts of air through the suspension of solids in the reactor. As stated above, the oxygen supplied must be available to the biomass. How effectively oxygen is made available is a measure of the economic success of the reactor.
Mindful of the foregoing considerations, the fact is that the cost of aerating a reactor effectively and efficiently requires a large expenditure of energy; and filtration through a membrane device requires a relatively high inlet pressure and high velocity of flow of concentrate through the membrane device; this requirement of high mass flow under elevated pressure in turn dictates high pump pressures, and high flow rates at elevated pressures which results in large pressure drops.
In particular, the high energy requirements for pumping a suspension of organic solids from a bioreactor through a membrane filtration unit, and using the energy of the concentrate stream from the unit to entrain oxygen from an eductor requires that the kinetic energy of the concentrate stream be used to draw in and disperse the required oxygen-containing gas stream. Such a configuration has been suggested in French application 2,430,451 to Lambert et al filed Jul. 4, 1978. The efficiency of the system is adversely affected because dissipation of the kinetic energy of the recirculating stream provides no positive energy contribution to the recirculating stream.
The high mass flow and kinetic energy of the recirculating stream in the '451 reference contributes so much energy to the system that efficient mixing in the reactor results simply because of the high contribution of fluid energy, minimal residence time, and without concern as to the establishment of a recirculating pattern. Further, since a characteristic of an eductor is that gas entrainment is limited by the mass flow of the recirculating stream and the resulting pressure drop generated in the eductor, under optimum conditions, one can typically only entrain less than about 1 volume of oxygen per 5 volumes of recirculating liquid, or, 1 volume of air per volume of recirculating liquid.
This physical limitation will be more readily understood by reference to an illustrative example wherein a 30 liter reactor is provided with a recirculation stream of 6500 liter/hr (6.5 m.sup.3 /hr) so that the residence time is 16 sec. Of this stream, 3500 liter/hr goes to a single eductor which entrains 3500 liter/hr of air. The inlet pressure of the recirculating stream into the eductor is 200 kPa gauge (30 psig). Though the membrane bioreactor system operates at low to medium pressure, in the range from about 100 kPa to 500 kPa, depending upon whether the membrane filtration device uses a microfiltration or ultrafiltraton membrane, a high mass flow of solids-containing concentrate is available for a recycle stream. This mass flow is sufficiently high (i) to provide enough liquid as is required per unit of air entrained, (ii) to provide sufficient mixing to ensure homogenization of the biomass, and (iii) to establish a preselected recirculation pattern in the bioreactor.
The relatively high cost of operation of the combination of a bioreactor and a membrane filtration device can be off-set with a "micronizer" (a particular form of an in-line microbubble generator) positioned so as to provide a directed recirculating jet (referred to as a "tail-jet") within the bioreactor.
In particular, operation of a membrane filtration device with a shipboard bioreactor requires accepting the likelihood of serious membrane flux decline, that is the rate per unit area of membrane surface through which permeate leaves. Such decline is typically due to insufficient oxygen being introduced to meet the respiration rate of the biomass, resulting in clogging of the membrane's pores. This problem of clogging suggested that the use of a microporous gas diffuser means (such as a porous metal annular element) was contraindicated because of the proclivity of a microporous element to be clogged by biomass.
The challenge to provide the proper amount of air to an aerobic reactor has been addressed in numerous references such as Wastewater Engineering pp 492-502, Metcalf & Eddy Inc. McGraw Hill 1979; Activated Sludge Process: Theory and Practice by J. Ganczarczyk, pp 133-153, Marcel Dekker 1983; Wastewater Treatment Plant Design pp 241-258, Water Pollution Control Federation, 1977; and a host of patent references.
Favored among devices for introducing air into an aerobic reactor are jet aerators, because of the high oxygen transfer they efficiently provide, but have restricted flexibility because of their design. Jet aerators are also referred to as ejectors, injectors, venturi nozzles, and eductors. Such devices introduce oxygen and water in a two-phase stream at a velocity high enough to provide requisite mixing within the reactor. The two-phase stream leaves the jet aerator in the form of a free jet (referred to herein as a "tail-jet"), which having penetrated a certain distance into the surrounding liquid, loses its energy and breaks up into clouds of bubbles. (See Sorption Characteristics of Slot Injectors and Their Dependency on the Coalescence Behaviour of the System, by M. Zlokarnik Chemical Engineering Science Vol 34, pp 1265-1271, 1979; and, Design Manual - Fine Pore Aeration Systems U.S. Environmental Protection Agency, Office of Research and Development, Center of Environmental Research Information, Risk Reduction Engineering Laboratory, Cincinnati, Ohio 45268, September 1989).
Though much of the requisite oxygen transfer takes place in a jet aerator before the tail-jet is ejected into the reaction mass, the oxygen in the two-phase stream must also be transferred to the biomass in the reactor, and this requires a substantial residence time. An eductor, as used in the French '451 application, by itself, does not provide adequate oxygen transfer for a shipboard bioreactor.
Efficient operation of a shipboard bioreactor at full load required introduction of auxiliary air, in addition to that provided by the micronizer. Such an auxiliary air stream provides economical macromixing (the motive force for adequate recirculation of the biomass). When auxiliary oxygen is introduced as compressed air, the compressor provides the energy for macromixing. Auxiliary oxygen is introduced in an auxiliary stream with recycle, using a jet aerator or eductor to introduce air, and only as much energy as is required to provide a recirculation rate of liquid which provides the necessary oxygen requirement. Pumping liquid is an economical way to provide efficient movement of the liquid.
Prior art devices relied upon the recirculation stream to provide the kinetic energy for entrainment of oxygen and mixing of the reaction mass. There was, and is, very little motivation to provide recirculation energy in a recycle loop by using the energy of air (oxygen and/or ozone) under pressure, which air is required to feed oxygen to the biomass.
Yet, in the preferred embodiment, such energy is derived from the air used, the energy being transferred through a combination of (i) the micronizer which is a "fine bubble aerator", and (ii) an auxiliary aerator.
The micronizer is an in-line porous element having through-pores which place its interior and exterior surfaces in open fluid communication, and the device is configured to provide a tail-jet. If the tail-jet has enough energy it can establish a recirculation pattern within the reactor. The micronizer is preferably located outside the reactor, and operated in the recycle loop in combination with the reactor and MF or UF membrane means in this novel shipboard MBR system, as will be described in greater detail below.
The auxiliary aerator is most preferably an "air-only" coarse bubble aerator, which is simply a porous cylindrical element with a closed end and an open end. The closed end and cylindrical wall of the element have large pores in the range from 1 mm to 5 mm in diameter. Compressed air is blown through the open end of the aerator and the energy of the air provides the motive force to establish a desirable recirculation pattern in the reactor.
When used in combination in a shipboard MBR, the micronizer and the auxiliary stream allow the effective use of either a MF and/or a UF membrane means to provide a permeate of acceptable quality. There has been no suggestion that any prior art system using a bioreactor to digest oily water, whether on land or at sea, might be effective with only a membrane filtration means, to make the separation of the activated sludge reaction mass from the permeate.